The present invention relates to an aluminum composite material having neutron absorbing power that is useful as, for example, a structural material of a transport container or storage container and so forth of spent nuclear fuel, and its production method.
Although boron (B) is an element that has the action of absorbing neutrons, only the 10B isotope, which is present at a proportion of about 20% in naturally-occurring B, is known to actually have said action. Alloys in which B is added to an Al alloy have been used in the past as structural materials having neutron absorbing action.
Ordinary melting methods have been employed in the case of producing such an alloy. Since the liquidus temperature rises rapidly as the amount of B added increases however, various methods are used, including adding B to the Al alloy in the form of a powder or Alxe2x80x94B alloy, adding B to an Al melt in the form of a borofluoride such as KBF4 to form an Alxe2x80x94B intermetallic compound, and using a casting or pressurized casting method starting at a temperature equal to or below the liquidus temperature at which both liquid and solid are present. However, various improvements have been made to enhance mechanical properties such as strength and ductility. There are numerous examples of these improvements, some of which include Japanese Unexamined Patent Application, First Publication No. Sho 59-501672, Japanese Unexamined Patent Application, First Publication No. Sho 61-235523, Japanese Unexamined Patent Application, First Publication No. Sho 62-70799, Japanese Unexamined Patent Application, First Publication No. Sho 62-235437, Japanese Unexamined Patent Application, First Publication No. Sho 62-243733, Japanese Unexamined Patent Application, First Publication No. Sho 63-312943, Japanese Unexamined Patent Application, First Publication No. Hei 1-312043, Japanese Unexamined Patent Application, First Publication No. No. Hei 1-312044 and Japanese Unexamined Patent Application, First Publication No. Hei 9-165637.
In Alxe2x80x94B alloy according to this type of melting method, when B is added that absorbs neutrons, intermetallic compounds such as AlB2 and AlB12 are present as B compounds, and when a large amount of AlB12 in particular is present, workability decreases. However, since it is technically difficult to control the amount of this AlB12, addition of the amount of B up to 1.5% by weight is the limit for practically used materials, and thus, neutron absorbing effects are not that large.
In addition, borals are materials other than the Alxe2x80x94B alloy according to the melting methods described above that have neutron absorbing action. This boral is a material in which a powder, in which 30-40% by weight of B4C is blended into an Al matrix material, is sandwiched followed by rolling. However, not only is the tensile strength of this boral low at about 40 MPa, since its elongation is also low at 1% making molding and forming difficult, it is currently not used as a structural material.
An example of a production method of Alxe2x80x94B4C composite materials that still leaves something to be desired involves the use of powder metallurgy. This method consists of uniformly mixing Al alloy and B4C both in the state of a powder followed by solidifying and molding. In addition to being able to avoid the above problems accompanying melting, this method offers advantages including greater freedom in selecting the matrix composition. In U.S. Pat. No. 5,486,223 and a series of following patents by the same inventor, a method is described for obtaining an Alxe2x80x94B4C composite material having superior strength characteristics using a powder metallurgy method. In particular, U.S. Pat. No. 5,700,962 focuses on the production of a neutron-blocking material. However, in these inventions, due to the use of a special B4C to which specific elements are added to improve binding with the matrix, the process is complex, and there were considerable problems in terms of cost for practical application. In addition, there were also numerous areas of concern with respect to performance, such as the occurrence of gas contamination as a result of heating and extrusion of a porous molded article in which the powder is solidified with CIP only, and significant deterioration of characteristics as a result of exposing to a high temperature of 625xc2x0 C. or higher during billet sintering depending on the matrix composition.
As described above, since there are limitations on the added amount of a compound having neutron absorbing power such as B in Al alloy produced with a melting method, the neutron absorbing effects were small. In order to resolve this problem, although numerous inventions have been made as mentioned above, in order to work those inventions, there were many prerequisites that considerably raised production cost, including melting a master alloy in which the ratios of internal compound phases (AlB2, AlB12 and others) have been controlled, and using extremely expensive concentrated boron, thus making these inventions difficult to apply practically at the industrial level. In addition, in terms of the operation, the working of these inventions with ordinary Al melting equipment has been nearly practically impossible due to problems such as contamination of the inside of the furnace (such as requiring that the furnace be washed to remove dross having a high B concentration, and contamination resulting from residual fluorides that were loaded into the furnace), and damage to the furnace materials caused by a high melting temperature (requiring a temperature of 1200xc2x0 C. and above in some cases).
In addition, a boral having a high B4C content of 30-40% by weight has problems with workability, preventing it from being used as a structural material.
In consideration of these background circumstances, in addition to seeking high neutron absorbing power by increasing the content of B, there has been a need for an aluminum composite material having neutron absorbing power, and its production method, that has superior mechanical properties such as tensile strength and elongation, is easily worked and can be used as a structural material.
Therefore, the object of the present invention is to provide an aluminum composite material having neutron absorbing power, and its production method, that enables the neutron absorbing power to be enhanced by increasing the B content, and is superior in terms of mechanical properties and workability.
In consideration of the present circumstances as described above, together with creating a method for inexpensively producing an Al composite material that satisfies the necessary neutron absorbing power and strength characteristics in the proper balance by using ordinary inexpensive B4C available on the market as an abrasive or refractory material, the inventors of the present invention found an alloy composition (including the amount of B4C added) in which the maximum effects of this method are demonstrated.
The present invention employed the following means to solve the above problems.
An aluminum composite material having neutron absorbing power of the present invention is characterized in that it contains in Al or an Al alloy matrix phase B or a B compound having neutron absorbing power in an amount such that the proportion of B is 1.5% by weight or more to 9% by weight or less, and that the aluminum composite material has been pressure sintered.
In this case, the B or B compound having neutron absorbing power contained in the Al or Al alloy matrix phase is preferably such that the proportion of B is 2% by weight or more and 5% by weight or less.
According to this aluminum composite material having neutron absorbing power, the amount of B or B compound added is high, and tensile characteristics and other mechanical properties are superior. In addition, its production cost can be held to a low level.
The production method of an aluminum composite material having neutron absorbing power of the present invention comprises adding a B or B compound powder having neutron absorbing power in an amount such that the proportion of B is 1.5% by weight or more to 9% by weight or less to an Al or Al alloy powder, and pressurized sintering the powder.
In this case, it is preferable to use a rapidly solidified powder having a uniform, fine composition for the Al or Al alloy powder, while boron carbide (B4C) particles are preferably used as the B compound powder. The mean particle size of the above Al or Al alloy powder is preferably 5-150 xcexcm, and B4C particles having a mean particle size of 1-60 xcexcm are preferably used as the B compound particles used.
In addition, hot extrusion, hot rolling, hot hydrostatic pressing or hot pressing, or any of their combinations, can be used as the method of pressurized sintering.
These pressurized sintering methods are all characterized by charging a powder into a can (canning) followed by drawing a vacuum while heating to remove the gas components and moisture adsorbed on the surface of the powder inside the can, and finally sealing the can. This canned powder is then subjected to heat processing while maintaining the vacuum inside the can.
Moreover, after performing the above pressurized sintering, heat treatment is preferably suitably performed as necessary.
According to this production method of an aluminum composite material having neutron absorbing power, by employing a powder metallurgy method using pressurized sintering, an aluminum composite material can be produced that has superior tensile characteristics and other mechanical properties even if the amount of B or B compound added is increased. Thus, an aluminum composite material can be provided that is able to improve neutron absorbing power while also having superior workability.